COPPER(I) DIOXYGEN REACTIVITY OF MONONUCLEAR COMPLEXES WITH PYRIDYL AND QUINOLYL TRIPODAL TETRADENTATE LIGANDS - REVERSIBLE FORMATION OF CUO2 = 1 1 AND 2/1 ADDUCTS/

Copper(I) complexes possessing a series of related tripodal tetradenta te ligands with pyridyl- and/or quinolyl-containing groups have been i nvestigated in reactions with dioxygen (O2). The ligand variations all ow for the testing of effects of ligand donor ability and steric facto rs. Copper-dioxygen complex stabilities, preference for formation of 1 :1 Cu-O2 and/or 2:1 Cu2-O2 adducts, Cu(n)-O2 (n = 1, 2), spectroscopic properties, and reactivity characteristics have been investigated, Th e ligands are L = tris(2-pyridylmethyl)amine (TMPA), bis(2-pyridylmeth yl)(2-quinolylmethyl)amine(BPQA), bis(2-quinolylmethyl)(2-pyridylmethy l)amine(BQPA), and tris(2-quinolylmethyl)-amine (TMQA). The 2:1 adduct [{(TMPA)CU}2(O2)]2+ (1c), with a trans-(mu-1,2-peroxo)dicopper(II) st ructure, has previously been shown to form reversibly from the interac tion of O2 with the copper(I) complex [(TMPA)-Cu(RCN)]+ (1a) (R = Me, Et) (Tyklar, Z.; et al. J. Am. Chem. Soc. 1993,115, 2677). The corresp onding Cu(I) complexes [(BPQA)Cu]+ (2a), [(BQPA)Cu]+ (3a), and [(TMQA) Cu]+ (4a) have been synthesized as either PF6- or ClO4- salts. They la ck an RCN nitrile ligand as isolated solids, but 2a-4a appear to coord inate nitriles in MeCN or EtCN solutions. The X-ray structure of 4a ha s been determined (triclinic space group P1BAR, a = 13.130 (2), b = 16 .570 (2), c = 12.956 (2) angstrom, alpha = 111.44 (1) beta = 98.49 (1) , gamma = 84.46 (1)degrees; V = 2592.1 (6) angstrom3; Z = 2), and it e xhibits a pseudotetrahedral coordination to the alkylamine and three q uinolyl N-donors. Copper(I) complexes 2a and 3a react with triphenylph osphine to form adducts [LCu(PPh3)]+ (2d, 3d). The X-ray structure of 3d (monoclinic space group C2/c; a = 16.341 (4), b = 19.017 (6), c = 2 6.860 (7) angstrom; beta = 105.48 (2)degrees, V = 8044 (4) angstrom3; Z = 8) indicates that one of the quinolyl arms of the BQPA ligand is u ncoordinated and dangles away from the Cu(I) ion, leaving a distorted tetrahedral coordination, comprising N(pyridyl), N(quinolyl), N(alkyla mino), and P copper ligation. Cyclic voltammetric measurements carried out on 1a-4a plus [(TMPA')Cu(CH3CN)]+ (1a'; TMPA' possesses a -C(O)Me ester group attached to the 5-position of one pyridyl ring of TMPA) s how that the Cu(II)/Cu(I) redox potentials can be greatly influenced b y these ligand variations. E1/2 values span a large range, from -0.61 to -0.24 V for 1a-4a, versus Ag/AgNO3 in dimethylformamide; the value for 1a' is -0.56 V. The BPQA complex 2a reacts rapidly with O2 at -80- degrees-C in EtCN solvent to give an intensely purple stable solution adduct [{(BPQA)Cu}2(O2)]2+ (2c), as evidenced by manometric O2-uptake measurements (CU:O2 = 2:1) and the UV-vis spectrum, which closely rese mbles that of 1c, with lambda(max) = 535 (epsilon = 10 500 M-1 cm-1), ca. 440 (sh, epsilon = 2000 M-1 cm-1), and ca. 600 nm (sh, epsilon = 7 600 M-1 cm-1). The chemical reactivity parallels that of lc; reaction with PPh3 gives 2d with evolution of O2, while exposure to H+ produces hydrogen peroxide in 94% yield, as determined by iodometric titration . The BQPA-containing complex 3a reacts to form a dark brown adduct [( BQPA)Cu(O2)]+ (3b), which is stable at -80-degrees-C in EtCN. Manometr y (Cu:O2 = 1:1) and the drastically different UV-vis characteristics w ith lambda(max) = 378 nm (epsilon = 8200 M-1 cm-1) attest to its formu lation as a 1:1 adduct, formally a Cu(II)-superoxide species. Reaction s of 3b with PPh3 and H+ proceed with formation of products analogous to those found with 2c, and phosphine adduct 3d could be isolated. Rea ction of 3b with the TMPA results in a ligand-exchange reaction, with production of peroxo species 1c, as followed by UV-vis spectroscopy at -80-degrees-C. The binding of O2 to 3a to give 1:1 adduct 3b is rever sible, as demonstrated by cycling experiments, i.e., the alternate and repetitive removal of O2 from 3b by application of a vacuum, followed by reoxygenation of 3a; this process was monitored spectrophotometric ally. Steric factors are seen to be important in this unique stabiliza tion of the Cu-O2 complex 3b. With the even more bulky TMQA ligand com plex 4a, no reaction with O2 is observed. Possible structures of the C u-bound O2 ligand in 1:1 adduct 3b are enumerated. Dioxygen binding ch aracteristics and variations observed for 1a-4a and 1a' are examined i n terms of electronic (i.e. redox) characteristics and steric effects and in light of recent complete kinetic-thermodynamic characterization of the O2-binding in 1a-3a (Karlin, K. D.; et al. J. Am. Chem. Soc. 1 993, 115, 9506). Steric and not electronic effects appear to predomina te in determining the O2-reaction chemistry. The biological relevance of 2:1 and 1:1 Cu:O2 adduct chemistry is also mentioned.